1. Field of the Invention
This invention relates to the field of measuring thermophysical properties of gases, and more particularly, to the determination of thermophysical gas properties using gross inferential properties and empirical correlations.
2. Description of the Related Art
Current technology provides two approaches to energy flow rate measurement for natural gas. The first requires a composition assay and a flow rate measurement. The composition assay allows calculating the heating value of the gas, and is also required to calculate selected gas properties (e.g., gas density) needed to determine energy flow rates. The second approach measures gas density and heating value directly, using special instrumentation, and requires no composition assay. Each approach can be further divided into sub-categories, based on the equipment used to effect measurements.
Gas chromatographs measure gas composition by separating the gas to be measured into pure components, and then detecting the concentration of each component separately. The process includes collecting a gas sample from the pipeline, and injecting it into one or more columns. After separation by the columns, the magnitude of pure gas component concentrations are sensed by various detectors. Gas chromatographs are typically quite expensive, require several minutes to effect an analysis, and require specially-prepared gas composition standards to calibrate the detectors for each targeted gas component. After the cost of instrument housing, calibration standards, sampling systems, and other accessories are added together, the total cost of purchase and maintaining a gas chromatograph system in the field is simply impractical for gas suppliers dealing in volumes of less than one million scfd.
Calorimeters provide a way to directly measure heating value, because they burn a gas sample and measure the heat generated. However, commercially available calorimeters measure standard volumetric heating value at low pressure, and not at flowing temperature and pressure. Thus, some measurement of gas density is also required to calculate the energy flow rate, even for volume-based meters. Reconditioned calorimeters can be purchased for approximately $10,000 to $20,000, but are difficult to procure as new items.
Another method of measurement involves the use of an inferential calorimeter, which infers heating value from stoichiometric combustion properties. However, a densitometer is still required to determine the energy flow rate for a volume-based meter. Periodic calibration is required using pure methane gas having a known heating value. To reach the required stoichiometric condition in one such device, the fuel flow rate (which is correlated to heating value) must be changed to accommodate rich gas (i.e., high heating value) and lean gas (i.e., lower heating value). Prices for these instruments also range from approximately $10,000 to $20,000.
There are also other devices, such as the PMI system manufactured by Badger Meter, Inc., which provide real-time, direct measurement of natural gas energy flow. Thus, these systems are not limited to measuring the energy content of the gas. However, such systems are used in conjunction with flow meters and sampling lines, such that the value measured must be scaled up to the pipeline rate using differential pressure measurements across the pipeline and sampling line orifices so as to exploit the thermodynamic similarity between the gas in the pipeline and the sampling line orifices. However, as a practical matter, the mini-orifice used is not geometrically similar to the actual pipeline orifice, and the flow characteristics between the two may be quite different.
Attempts have also been made to correlate standard volumetric heating value to the speed of sound in gas. However, there is no published evidence of attempts to correlate sound speed with the actual volumetric heating value, which depends on the gas composition, flow temperature, and pressure. Thus, correlation requires the use of known diluent concentrations, which may be unavailable in the field.
Other correlation attempts include measurement of gas dieletric constants, thermal conductivity, specific heat and other properties. While the results are encouraging, error values may be as high as 3.5% and no published data exists to verify measurement and prediction of the actual volumetric heating value from sensed properties at various operating pressures.
In summary, the state of the art in natural gas flow rate and energy flow rate measurement requires determination of a detailed flowing gas composition analysis. The composition assay is then used to calculate gas properties needed to determine the energy flow rate for a particular pipeline, and currently requires the application of an expensive and technically sophisticated instrument, in the form of a gas chromatograph.
A low-cost, easy-to-maintain apparatus and method are therefore required to facilitate accurate energy flow rate determinations for natural gas and other fluids. The need for such device increases as a result of industry deregulation, which has introduced widely-varying compositions of gas into natural gas pipelines.
The system and method of the present invention provide for determination of thermophysical properties of multi-component gases based on the determination of two quantities: the speed of sound in the gas and the concentration of a plurality of components comprising the gas. As a specific example, the concentration of carbon dioxide and nitrogen are determined, along with the speed of sound in the gas, to determine a thermophysical property (e.g. the Mixture Molar Ideal Gross Heating Value) as a function of an empirical correlation between the thermophysical property, the speed of sound, the concentration of carbon dioxide, and the concentration of nitrogen in the gas. For greater accuracy, the speed of sound may be determined at a fixed temperature and pressure of the gas.
Depending on the gas components for which the concentration is determined, various thermophysical properties can be determined more or less accurately. For example, the Mixture Molar Ideal Gross Heating Value, the Mixture Molecular Weight, the Mass-Based Heating Value, and the Density of the gas can all be determined within about xc2x10.02% of selected model values by implementing the system and method of the present invention.
The speed of sound and concentration of gas components may be determined directly (e.g., via measurement), or indirectly. For example, the concentration of a particular gas component may be determined by correlating a thermodynamic property for the selected component with one or more directly measurable inferential properties of the component.
The invention also includes a method to determine a gas thermophysical property comprising the steps of determining a speed of sound in the gas, determining a plurality of gas component concentrations which make up a subset of the total number of components comprising the gas, and then determining the selected thermophysical property as a correlation function between the thermophysical property, the speed of sound in the gas, and the plurality of gas component concentrations, wherein the number of component concentrations is less than the total number of total components comprising the gas. Examples of component concentrations which may require determination include those for carbon dioxide and nitrogen; the speed of sound may be measured at a fixed temperature and pressure for greater accuracy.
As is the case for the apparatus, the method may be used to determine the Mixture Molar Ideal Gross Heating Value, the Mixture Molecular Weight, the Mass-Based Heating Value, or the Density of the gas. The speed of sound and component concentrations may be measured directly, or indirectly. Thus, the concentrations may also be determined by correlating a thermodynamic property of selected components with one or more directly measurable inferential properties of the same components.